As is known, LEDs are more and more used in lightening devices (lamps) in increasing fields, due to their advantageous characteristics as to costs, dimensions, duration, directionality and electrical efficiency, so that the LED lamp market is projected to grow by more than ten-fold over the next decade.
LED based lamps are used both stand-alone and included in more complex systems. In the latter case, often a controller is configured to manage the operation of a number of different loads. For example, in the automotive field, control of the switching of the LEDs and their functionality is generally included in a system. The system includes a microcontroller and at least one drive device that are formed in different chips for controlling a number of functions, including, e.g., mirror adjustment, lock control, direction indicator, various lightening functions.
An example of such a system is shown in FIG. 1. Here a microcontroller 1 has a plurality of controller I/O pins 1A coupled, through a number of respective connection lines 2, e.g., a Serial Peripheral Interface bus, to a drive device 3. The drive device 3 has a first plurality of drive I/O pins 3A coupled with the connection lines 2, a second plurality of drive I/O pins 3B coupled with external loads 4 (including, e.g., motors for mirror adjustment, mirror folding, door locking, not shown in detail) and a third plurality of drive I/O pins 3C coupled with a plurality of LED groups 5, for example ten. Each LED group 5, in turn, generally includes a plurality of LED elements 7 series-connected to a respective resistor 6.
The drive device 3 generally includes an interface, logic and diagnostic unit 10 coupled to the first plurality of I/O pins 3A for communication/data exchange with the microcontroller 1. The interface, logic and diagnostic unit 10 is also coupled, through respective driver elements 8, to the second plurality of drive I/O pins 3B and, through respective power devices 11, e.g. high-side MOS transistors, to the third plurality of I/O pins 3C. A supply voltage VB is fed to the blocks of the drive device 3, including the interface, logic and diagnostic unit 10, the driver elements 7 and the power devices 11.
With the architecture of FIG. 1, the LED groups 5 are generally switched on and off by the interface, logic and diagnostic unit 10 according to a PWM modulation technique to control light brightness. In fact, from a physical point of view, LED brightness is correlated to the current flowing through them and, varying the average current flowing in the resistors 6 through a PWM modulation of the supply voltage applied thereto, it is possible to adjust the brightness according to the requirements.
To this end, generally, the power devices 11 are supplied according to a standard duty-cycle, in case adapted to the specific type and number of LED elements 7, as stored in the interface, logic and diagnostic unit 10.
In many applications, it is desired to maintain a constant light brightness when the LED elements are on. Brightness of current LEDs depends on a number of parameters, including actual supply voltage level. However, in particular in automotive applications, supply voltage is not generally constant. In fact, in the automotive field, numerous voltage transients may occur on the supply voltage VB, both negative and positive caused, for example, by start of a vehicle engine, which may cause a drop of the supply voltage VB to a half of its nominal value (e.g., from 12 V to 6 V), and switching on/off of heavy inductive loads, such as window opening motors. Therefore, in case of varying supply voltage, brightness is not constant, and flickering may occur, which is undesired.
To avoid this nuisance, the microcontroller 1 may modify the standard duty-cycle of each of the LED groups 5 so as to maintain a constant brightness in case of varying supply voltage. In particular, in presence of supply voltage variations, the microcontroller 1 may calculate correction factors of the LED duty-cycle and send suitable control signals to the drive device 3.
With the above approach, a direct drive input from the microcontroller 1 to each LED group 5 would be necessary; therefore, both on the microcontroller 1 and on the drive device 3 a plurality of dedicated pins would be needed. This would entail an increase of the number of pins which is often not possible and, in any case, undesired.
Even when the existing lines 2 are capable to manage the brightness correction signals, e.g. by being implemented as an SPI bus, the frequent variations of the supply voltage would cause a high signal traffic on the lines 2, which is disadvantageous.
In addition, a high job load for the microcontroller 1 is generated, which may be problematic on account of the further control functions carried out by the microcontroller 1.
Moreover, since in standard systems the communication speed on the Serial Peripheral Interface bus implementing the lines 2 and relevant interfaces in the microcontroller 1 and in the drive device 3 is limited, frequent duty-cycle adjustment may be not always possible in real time, preventing maintaining the desired constant brightness.
Furthermore, since control signal are associated with electro-magnetic emissions, a high traffic due to frequent duty-cycle adjustment may generate a high EMC noise, which is undesired.
Another solution may reside in measuring and controlling the current flowing in the LED groups. However, this solution would involve a high power dissipation inside the power drive device.